[OMEMO] Set access_model=open on PEP elements

OMEMO plugin should set pubsub#access_model to open for eu.siacs.conversations.axolotl.devicelist and eu.siacs.conversations.axolotl.bundles PEP nodes to allow using OMEMO between non-subscribers (e.g. in multiuser groups).

If devicelist node already exists and not configured with pubsub#access_model, publishing to it with publish-options could return precondition-not-met. Gajim should reconfigure this PEP node first.